South Korea’s Yonhap News Agency on Tuesday quoted lawmakers briefed by South Korea’s National Intelligence Service (NIS) as saying that the work was taking place at the Tongchang-ri launch site and involved replacing a roof and a door at the facility.

Satellite images seen by 38 North, a Washington-based North Korea thinktank, showed that structures on the launch pad had been rebuilt sometime between 16 February and 2 March.

The news comes days after a summit on denuclearisation in Hanoi between Trump and North Korean leader Kim Jong-un broke down over differences on how far North Korea was willing to limit its nuclear program and the degree of US willingness to ease sanctions.

President Donald Trump’s national security adviser, John Bolton, said on Tuesday that the United States would look at ramping up sanctions on North Korea if Pyongyang did not scrap its nuclear weapons program.

Bolton told Fox Business Network that Washington would see whether Pyongyang was committed to giving up its “nuclear weapons program and everything associated with it.”

“If they’re not willing to do it, then I think President Trump has been very clear ... they’re not going to get relief from the crushing economic sanctions that have been imposed on them and we’ll look at ramping those sanctions up in fact,” said Bolton, a hardliner who has advocated a tough approach to North Korea in the past.

Asked to comment, the White House referred to the US state department, which did not immediately respond. A US government source said the NIS was considered reliable on such issues, but added that the work described did not seem particularly alarming, and certainly not on a scale of resuming missile tests that have been suspended since 2017.

Trump told a news conference after an unprecedented first summit with Kim on 12 June in Singapore that the North Korean leader had promised that a major missile engine testing site would be destroyed very soon.

Trump did not identify the site, but a US official subsequently told Reuters it was the Sohae satellite launching ground, which is located at Tongchang-ri.

Kim Jong-un also pledged at a summit with South Korean President Moon Jae-in in September to close Sohae and allow international experts to observe the dismantling of the missile engine-testing site and a launch pad.

Signs that North Korea had begun acting on its pledge to Trump were detected in July, when a Washington thinktank said satellite images indicated work had begun at Sohae to dismantle facilities.

However, subsequent images indicated North Korea had halted work to dismantle the missile engine test site in the first part of August.

The breakdown of the summit in Hanoi last week has raised questions about the future of US-North Korea dialogue.

Mike Pompeo, US secretary of state, said on Monday he was hopeful he would send a delegation to North Korea in the coming weeks but that he had had “no commitment yet”.

While North Korea’s official media said last week Kim and Trump had decided at the summit to continue talks, its vice foreign minister Choe Son Hui told reporters Kim “might lose his willingness to pursue a deal” and questioned the need to continue.

US state department spokesman Robert Palladino told a news briefing that the United States remains “in regular contact” with North Korea, but he declined to say whether they had been in contact since the summit.

Yonhap also quoted lawmakers briefed by intelligence officials as saying that the five-megawatt reactor at North Korea’s main nuclear site at Yongbyon, which produces weapons-grade plutonium used to build bombs, had not been operational since late last year, concurring with a report from the U.N. atomic watchdog.

The fate of the Yongbyon nuclear complex and its possible dismantling was a central issue in the Hanoi summit.

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